1. Field of the Invention
The present invention is broadly concerned with new processes for the formation of low-density, high surface area gel products. More particularly, the invention is directed to such methods and the novel gel products themselves, wherein the gel products are formed in an enclosed chamber containing a mixture including particles of material suspended in gas under conditions to cause the particles to aggregate within the chamber and form a gel. Products may be produced having unprecedentedly low densities below 3.0 mg/cc. Gel products can be formed from virtually any starting material, provided that appropriate gelling conditions are established within the chamber.
2. Description of the Prior Art
Aerogels are a class of solid materials, produced through the sol-gel process, that are generally characterized by a fragile skeletal structure defining highly-accessible, branched mesopores. In contrast to other mesopore materials, aerogels represent a very unique and exciting class of solid materials exhibiting amorphous structures, extremely low apparent densities (with up to 95% of their volume occupied by air), high inner surface areas, and the potential to be formed into monoliths. Husing and Ulrich present a working definition of aerogels as “. . . materials in which the typical structure of the pores and the network is largely maintained while the pore liquid of a gel is replaced by air.” Aerogels have very unusual properties. For example, SiO2 aerogels have a high transparency that is close to that of glass, a thermal conductivity corresponding to that of polystyrene or polyurethane foams, and very high specific surface areas. The unique feature of aerogels is the combination of these physical properties in one material. Aerogels are made via a liquid phase sol-gel process. In order to remove the solvent liquid from the pores of the wet gel (produced during the sol-gel process) without damaging the fragile skeletal network, very special drying techniques must be employed. This is typically accomplished through a supercritical drying process, in which the wet gel (immersed in the solvent material) is placed and sealed in a pressure vessel (autoclave). The temperature and pressure of the autoclave are then increased and adjusted to a point above the critical point of the solvent. The solvent material is then vented out of the pressure vessel while holding the temperature constant. Although significant shrinkage of the network typically occurs during the supercritical drying process, the remaining 3-D solid aerogel monolith remains structurally intact, Regardless of the specific processes used, the current state of the art of aerogel production is critically dependent on the liquid-based sol-gel process and, more importantly, the complex supercritical drying process necessary to remove the gel liquid without damaging the network structure.
Silica aerogels are by far the most well-developed and extensively studied of all aerogel materials. Traditionally, silica aerogels are produced via a base-catalyzed reaction of TMOS (tetramethoxysilane) or TEOS (tetraethoxysilane), usually with ammonia as the catalyst. Once gelation is complete, the resulting liquid in the wet gel is then removed via supercritical drying. The type and concentration of the precursors, the relative concentrations, the type of solvent, the temperature, and the pH of the sol-gel process all have a definitive effect on the resulting structure and properties of the silica aerogel.
A relatively new class of aerogel materials, organic aerogels, are formed through the polymerization of resorcinol/formaldehlyde (RF) or melamine/formaldehlyde (MF) precursors via the sol-gel process, followed by supercritical drying. The key variables determining the structure and properties of organic aerogels are the catalyst concentration and the pH of the solution. Carbon aerogels are then prepared by pyrolysis of organic aerogels in an inert gaseous environment at temperatures ranging from 600 to 1100° C., producing a solid carbon aerogel monolith. Carbon aerogels represent a unique and exciting class of aerogel materials due to the fact that they are the first electrically conductive aerogel materials. For example, carbon aerogels have densities (mg/cm3) of 100-600, surface areas (m2/g) of 400-1,000, average pore sizes (nm) of 4-30, and electrical conductivities (W/cm2) of 1-10.
The excellent electrical conductivities of carbon aerogels, along with their high inner surface areas, make them candidates for electrodes in electrical and electro-chemical applications. Currently, one of the most promising of these applications is in the development of electro-chemical double-layer capacitors (EDLCs), also known as supercapacitors or ultracapacitors. These devices are characterized by moderate energies and high power densities and are used in such applications as backup power supplies and on-demand peak power sources, where it is required to reversibly capture a large quantity of electric charge. Due to their high inner surface areas and highly interconnected network structures, carbon aerogels are currently the most promising new material for this application.
In addition to their superior electrical conductivity properties, carbon aerogel materials also show great promise in certain applications as thermal insulators. Although materials such as silica aerogels have long been identified as perhaps the best thermal insulators available, carbon aerogels are also very attractive in this arena. Total thermal conductivity is comprised of solid, gaseous, and radiative conductivities. The extremely low overall thermal conductivities of all aerogel materials is due partly to their high pore contents causing their solid thermal conductivities to be very low. On the other hand, the very small sizes of their pores cause their gaseous conductivities to also be quite low. Black (or highly opaque) carbon aerogels, in contrast to their silica-based counterparts, have very high IR extinctions and therefore possess much lower radiative thermal conductivities. This thermal feature of carbon aerogels makes them prime candidates for a wide variety of thermal insulation applications.
In 1998 Sorensen and coworkers demonstrated for the first time that aerosols could gel. The system was a simple acetylene/air diffusion flame in which the carbonaceous soot formed a macroscopic gel network. Soot is composed of ca. 50 nm spherical monomers, or primary particles. In a flame, these particles are at a high number density so that aggregation to fractal aggregates occurs rapidly. These aggregates form by a process called diffusion limited cluster aggregation (DLCA) and have a fractal dimension of D>1.8. Such aggregates are usually the final product in most flames, but Sorensen et al. showed that the heavily sooting acetylene flame had a volume fraction of soot roughly two orders of magnitude higher than flames for most other fuels. Thus they concluded that the rate of soot growth in the acetylene flame was five orders of magnitude faster, fast enough to form a gel in the flame.
Background references describing low-density aerogels include: U.S. Pat. Nos. 4,150,101; 5,313,485; 5,601,938; 6,296,678; 6,485,805; and 7,005,181; U.S. Published Patent Applications Nos. 2003/0022389; 2004/0029982; 2004/0159849; 2005/0064279; 2005/0131163; 2006/0116463; and foreign references EP 884376; JP 09202610; JP 2000265390; JP 2001072408; WO 2004/009673; and WO 2005/045977.